Watchdog (BORAXpress)

Description and default configuration

An external watchdog timer (WDT), Maxim MAX6373[1]), is connected to the PORSTn signal. During normal operation, the microprocessor should repeatedly toggle the watchdog input WDI before the selected watchdog timeout period elapses to demonstrate that the system is processing code properly. If the μP does not provide a valid watchdog input transition before the timeout period expires, the supervisor asserts a watchdog (WDO) output to signal that the system is not executing the desired instructions within the expected time frame. The watchdog output pulse is used to reset the μP.

Default mounting option is depicted in the following figure.

Watchdog timer default mounting option

WDI is connected to Zynq's PS_MIO15_500 I/O. This signal is available on Bora Xpress connectors as PS_MIO15_500 (J1.133).

MAX6373 timeout is pin-selectable. It can be configured through the WD_SET0 (J2.100), WD_SET1 (J2.98) and WD_SET2 (J2.96) signals. By default, they are configured as follows:

WD_SET2 = 1

WD_SET1 = 1

WD_SET0 = 0

This set selects the option (the exhaustive list of configurations options is descripted in table 1 of reference [1]):

tDELAY = first edge

tWD = 10s.

In other words, WDT is started when the first transition on WDI input is detected. Once started, its timeout period is 10s. The first transition of WDI input is under software control.

When the watchdog is started, the software (bootloader/operating system) must take care of toggling the watchdog trigger pin (WDI) before the timeout expiration.

Selecting different configurations

Since WD_SETx signals are routed externally, WDT configuration can be changed by optional circuitry implemented on the carrier board. Different solutions can be implemented on the carrier board, depending on system requirements. The easiest circuit consists of additional stronger pull-up/down resistors connected to WD_SETx pins in order to overrule default configuration. As MAX6373 allows to change the configuration during operation, more complex solutions can be implemented as well.

It is also worth mentioning that Zynq integrates a System Watchdog Timer (SWDT) that can optionally generates a reset pulse on PS_MIO15_500 pad if this is configured as SWDT reset. In case such a configuration is of interest, on request MAX6373 may not be populated. For more details about this option, please contact Sales Department.